Dozens of 8-bit microcontrollers (MCUs) inhabit the average new automobile, which says something about both 8-bit MCUs and automobiles. Today's automobiles are nothing like their predecessors. Whereas automobile engineers/ inventors were originally concerned with creating a means of transportation, today's engineers are now perfecting how our vehicles transport us, thanks to the microchip. The intelligence built into today's vehicles enables a safer, more comfortable and enjoyable experience than vehicles have ever offered. This evolution has created a demand for electronic systems unlike any other industry, and because of this demand, 8-bit MCUs have thrived.
The match between these MCUs and automotive applications is ideal for many reasons. It turns out that those reasons will make an even more compelling match for the foreseeable future–pushing more MCUs into cars, trucks, and even motorcycles and continuing the vehicles” evolution.
Why will 8-bit MCUs continue to be popular in this evolving industry? Largely because they offer a wealth of features at attractive prices. Also, the trends driving automotive electrical systems will prove to be key in the success of 8-bit devices.
Although it's often said that automotive applications can include only mature or less-advanced devices, some of these mature devices incorporate an abundance of innovative features. They may be small but 8-bit MCUs aren't simple. Strengths of 8-bit MCUs include:
- Integrated features. Eight-bit MCUs can incorporate a large variety of sophisticated peripherals, such as internal high-speed clocks and timers, integrated controller area network (CAN) and local interconnect network (LIN) buses, and many other features to increase the MCU's capabilities.
- Performance. With clock speeds into the tens of megahertz and peripherals such as hardware multipliers, 8-bit MCUs offer an increasing amount of processing power compared with previous generations.
- Reliability. These devices have earned their reputation for high reliability with many features that help ensure continuous and correct operation, so much so that 8-bit MCUs act as watchdogs or monitors for many critical applications.
- Flash memory. Mask ROM has been replaced by flash memory in most 8-bit MCUs, making 8-bit MCUs more versatile in embedded applications. Also, the range of program sizes can now include as much as 128KB of flash memory, allowing much more flexibility in a design.
- Optimized footprints. If there is the need for reduced functionality, 8-bit devices can offer a variety of small package sizes.
- Low power consumption. Despite their effective processing power, these MCUs provide many ways to manage power consumption so that applications consume no more power than is absolutely necessary. For functions that must remain active when the ignition is off, battery conservation is a strong advantage to using 8-bit MCUs. In fact, new 8-bit flash devices have current consumption comparable with traditional mask ROM devices.
- Low electromagnetic interference (EMI). Eight-bit MCUs easily meet the Society of Automotive Engineers' (SAE) EMI requirements in most cases by running at relatively low speeds.
- Low cost. Compared with 16- and 32-bit devices, 8-bit MCUs offer significant cost savings.
Some of these factors, notably increased performance and flash sizes at lower cost, have happened because reinvestment in 8-bit MCUs in recent years has rejuvenated these devices with the latest semiconductor process technologies and peripherals.
Leveraging the choices
When the time comes to choose a device, system designers have to consider all of their requirements: meeting system requirements from the customer, the form factor specs from mechanical designers, memory size from software developers, and many other specs for function, flexibility, efficiency, cost, and so on. When comparing devices and features, having a firm grasp of the choices available and the knowledge of how they apply to application requirements is critical.
The following features of 8-bit MCUs should factor heavily in a designer's decision:
- Their ability to reduce design complexity
- The capability to operate with low power
- The option for fast wake-up timing
- The integration of complex peripherals
- Reliability features such as watchdogs
- The inherent benefits of all-flash devices
- The associated cost savings
Because 8-bit MCUs integrate so many features, they offer the overall advantage of design simplicity. With high-speed internal clocks reaching into the multi-megahertz range, it's possible to use an MCU with a single resistor as the only external component. While an internal oscillator doesn't have the timing tolerance necessary for driving a CAN interface, the clock does provide enough stability for LIN. Thus, an MCU with barely any additional components can act as a full-capability networked slave device.
Internal clocks also offer the ability for lower current consumption and faster wake-up time. Take an application such as a vehicle”s wireless receiver. This module must go to a low-power sleep mode and wake up periodically to retrieve data from wireless sources, such as the tire-pressure sending units or remote keyless entry unit. An internal oscillator allows the MCU to enjoy extremely low standby current, while also allowing the MCU to wake up quickly to perform the necessary tasks. Additionally, because some MCUs can wake in just a few microseconds, it”s practical to service many interrupts from a sleeping start.
In addition to the internal high-speed clocks, 8-bit MCUs also integrate high-speed CAN modules and LIN-enhanced UARTs for networked applications. The advantage of these on-chip modules is that they can reduce the CPU load by performing many of the basic functions required by the networking protocol. This gives designers more freedom to integrate functionality into the MCU, since bandwidth is not lost on network handling.
For security and reliability, watchdog timers are critical for automotive applications, as they help prevent software runaways. A new addition to some of these timers is a windowing feature that provides even greater protection. You can set the timer to function like a traditional watchdog with a 100% open window or you can select a window of 75%, 50%, or 25% of the time, at which point the watchdog timer accepts a clearance only during the open period. This feature makes it far less likely that a looping program can periodically clear the watchdog timer and keep looping forever.
Another highly useful feature for automotive applications is flash memory. Since error-correction coding eliminates single-bit failures, and self-programming, along with secure boot loaders, enables updates in the field, flash offers the reliability and versatility required by a wide range of applications. A high degree of sector granularity allows quick updates of small memory areas, making updates and calibrations in the field a speedy task. During development, flash devices offer simpler and easier ways to reprogram the devices, often with just a few seconds necessary to rewrite an entire program. Additionally, flash technology is at a stage where it's possible to emulate traditional EEPROM in flash, thus in many cases eliminating the need for an external component.
While flash memory offers the ultimate in programming flexibility, it also provides safeguards against security risks. Eight-bit MCUs generally provide security settings that protect flash contents as needed. For example, you can combine security flags for write-prohibit and chip-erase–prohibit functions to get security equivalent to that of conventional ROM products. To protect from code theft, flash devices can implement security keys that keep unwanted persons from reading the flash program.
While advancements in 8-bit MCUs have led to new strengths, another recognized benefit of these devices is that they are often derived from a well-established family of device cores. This lineage offers a wealth of knowledge and expertise along with a familiar environment, which takes advantage of engineers' experience and enables them to respond quickly to changing requirements.
Of course, the driving factor in any design is usually cost. As mentioned, reinvestment in 8-bit MCUs has increased the cost competition between 8-bit MCUs and the rest of the MCU market. As feature sets increase, these devices are better able to compete against larger devices while maintaining their cost benefits. Also, tools for 8-bit devices are becoming more powerful and increasingly straightforward to use. The price of typical full-function emulators are now a fraction of what they were just a few years ago, and low-cost emulators continue to add features, making them excellent tools for a developer as well.
The road ahead
As we've explained, 8-bit MCUs have excellent characteristics that have contributed to their success. The main reason, however, for the recent upsurge in 8-bit MCU growth is because of low-cost networking standards in automobiles. These networks have simply moved 8-bit MCUs from their role as main processors to that of subprocessors used for handling a huge range of individual functions, as illustrated in Figure 1.
Figure 1: Body and security functions
Certainly a single, high-powered controller is ill-equipped to handle large numbers of applications. The necessary number of I/Os, high-current wires, and timing issues suggests that individual distributed MCUs are more realistic.
Individual control modules based on 8-bit MCUs also make it easy to differentiate car models. To upgrade a model, simply add several automatic features by plugging in the appropriate modules. Additionally, the modules are easy to replace in the event of failure, simplifying service.
Eight-bit MCUs also turn out to be ideal platforms for adding new components to an automobile. Want to implement a new personal comfort feature? Control it with an 8-bit MCU. The enormous growth in such features has given increased opportunity to 8-bit MCUs.
Still, some degree of integration makes sense and can be expected in the years to come. Since the purpose of integration is to pull features from what were previously subsystem controllers into a single main controller, the opportunities for 8-bit MCUs will change. In an integrated system, the “satellite” modules will have a more narrowed function and, therefore, require fewer features than previously needed. This will create an opportunity for new products based on 8-bit MCUs.
For example, one approach is to combine them with other devices in multichip modules. As shown in Figure 2, the example on the left is a straightforward combination of an 8-bit MCU with a LIN transceiver/voltage regulator chip. This module works directly on 12V battery power and handles applications such as sideview mirror control or window lift. The example on the right combines a MCU with a stepper-motor controller. An 8-bit MCU could be combined with a number of additional components for a variety of applications.
Figure 2: Multi-chip package solutions
Therefore, as the means for enabling both the distributed and integrated approaches, networking standards continue to evolve. Because the LIN standard requires only a single wire compared to CAN's two, LIN is becoming the low-speed, low-cost network of choice for automotive body applications that can encompass the entire vehicle. A general hierarchy of networks is emerging, with actuators and sensors at the periphery, MCUs interfacing to them via CAN or LIN, and gateway devices routing messages over different domains and networks.
Whether integrated or distributed architectures prevail, both trends generally point toward greater use of 8-bit MCUs. Versatility, capability, and low cost make these devices a good match for automotive applications.
Connecting the dots
Eight-bit MCUs are familiar because they were the original success story for microprocessors in cars. They”ve been around so long that observers could be forgiven for thinking that they should have disappeared years ago in favor of 16- or 32-bit processors. Due to the ever-growing list of features available on 8-bit devices, especially integrated peripherals and flash memory with error-correction coding, combined with straightforward automotive networking standards such as CAN and LIN, 8-bit MCUs have spread out across the typical car in increasing numbers and will continue to do so for years to come.
Adam Prengler is a technical applications engineer at the Automotive Strategic Business Unit of NEC Electronics America, Inc.
Dave Stone is a technical marketing manager at the Automotive Strategic Business Unit of NEC Electronics America, Inc. Both authors can be reached at .